1. Field of the Invention
The present invention relates to a friction stir welding method and a friction stir welding device for welding an object composed of a plurality of members to be welded by stirring the object in a solid phase
2. Description of the Related Art
As a conventional method of welding a plurality of members to be welded, a friction stir welding method is available. In the conventional friction stir welding, a welding tool is arranged on one side of an object whose members to be welded are stacked. Next, the welding tool is rotated and pressed against the object so as to be inserted into the object. The welding tool makes the members to be welded partially fluid in the non-molten state by frictional heat and stirs the fluidized members so as to weld the members, as disclosed in, for example, Japanese Patent Laid-Open Publication No. 2001-314983.
In the conventional friction stir welding method, the welding tool is rotated and then inserted into an object. However, when a welding device is low in its accuracy and rigidity, the rotating tip of the welding tool is shifted from the precise position, causing the so-called whirling. In this case, the welding tool is inserted into the object at a position out of the target welding position which is originally planed. As a result, a problem arises that the welding tool cannot be inserted precisely into the target welding position.
Further, there is a case that a plurality of parts of the members to be welded are sequentially spot-welded. As mentioned above, in the conventional friction stir welding method, the insertion position of the welding tool may be shifted from the target welding position. Therefore, the interval of actual insertion positions cannot be set to a predetermined interval, thereby, a problem arises that the welding strength may be varied. Further, when the variation between the target welding position and the actual insertion position is large, the welding strength may be decreased. Further, since the actual insertion position is shifted from the target welding position, the beauty of the welded object may be deteriorated.